Radiation source



Jan. 18, 1955 1R. B. JACOBS ET AL RADIATION SOURCE Filed Sept. 26, 1952 $55 VI. mm 5 M M M mXVm w. .5 0 0 A MM V m United States Patent "()fiice I 2,700,111 Patented Jan. 18, 1955 RADIATION SOURCE Application September 26, 1952, Serial No. 311,734

6 Claims. (Cl. 250106) This invention relates to radioactive sources and to a method and apparatus for making the same. More particularly, the invention is concerned with an improved adioactive source that emits beta particles from two aces.

It is an object of this invention to provide a new and improved method and means for supporting a radioactive source. Another object of this invention is to provide a radioactive source which is simple in construction and yet durable. A further object is to provide an encased radioactive source adapted to emit useful radiation in more than one direction. Still another object of our invention is to provide an apparatus adapted to produce such an encased radioactive source. These and other objects of the invention will become apparent as the description thereof proceeds.

Briefly, in accordance with this invention, the source is made by depositing a solution of radioactive salt on a thin base sheet, the radioactive material, such as strontium 90, being confined Within a limited area on the base sheet. A second or cover sheet is used to enclose the radioactive material. The laminated unit is mounted in a split holder having registering apertures on opposite faces of the holder to provide a double-faced source unit.

For a more complete understanding of this invention and of the objects and advantages thereof, the invention will be described in connection with the accompanying drawings wherein:

Figure 1 is a section of a preferred embodiment of the radioactive source according to this invention;

Figure 2 is a top view of the embodiment in Figure 1;

Figures 3 and 4 illustrate a shielding and manipulating apparatus which may be employed in utilizing the radioactive source unit according to this invention; and

Figure 5 is a perspective view'of one form of a jig used in depositing the radioactive material on the base sheet.

A deposit of radioactive material is placed on a thm base sheet, such as gold foil or mica, transparent to the spectrum of radiation from the radioactive material. A second sheet of similar material is bonded to the base sheet over the radioactive deposit by means of a plastic material. The base sheet and the cover sheet with the radioactive deposit between are placed in a shielding casing made of some material such, for example, stainless steel or brass. The shield or casing has a pair of opposed apertures or windows in register with the radio active deposit to permit equal radiations to be emltted 1n opposed directions from the same source. When not in use the casing containing the source is enclosed by a removable shutter adapted to bring a pair of shield plugs in register with the windows.

In Figures 1 and 2 we have shown a preferred form of our source assembly 10. It comprises a pair of annular discs 11 and 12 having an outside diameter of 1.25 inches and an inside diameter of 0.375 inch. These discs are ordinarily thin and may be about .062 inch thick. The radioactive source 13 per se is deposited upon a thin sheet, such as gold leaf or mica, comprising a disc 1.25 inches in diameter. The base and cover sheets 14 and 14a are about 0.004 inch thick and the radioactive source 13 may be deposited on one or both of these sheets employing the apparatus shown in Figure 1. The second sheet 14a is placed over the firsrt sheet 14 with a thermoplastic material 15 between the two sheets. One form of plastic material is a polyethylene sheet of 1%; inches diameter and having a central opening about inch in diameter to accommodate the radioactive source 13 which has been deposited on the sheet 14.

The two discs 11 and 12 with the intermediate sandwich unit of mica sheets 14 and 14a and polyethylene sheet 15 are brought together by means of screws 16. The entire unit 10 is then heated to a temperature of about 400 F. causing the polyethylene material to flow and make a tight seal between the mica sheets.

The above construction provides a uniform source of beta rays which are emitted in opposite directions through the windows 11a and 12a in the protecting discs 11 and 12. This source may be employed in the instrument described in our copending application S. N. 285,348.

Referring to Figures 3 and 4, we have illustrated one arrangement of the source unit 10 and a shutter mechanism which can be used with it. A hand-operated heavy metal shutter 20 is in gravity-contact with the cam 21, carried by shaft 22. This shutter 20 can be lifted around the source 10 and may comprise a pair of plates 23 and 24, a bottom block 25, and tungsten alloy inserts 26 mounted in the lower portion of the slots 27 in the side walls 23 and 24.

Shutter 20 can be raised by means of cam 21 actuated through rod 22. By rotating the cam 21 it acts on the bottom of the shutter unit 20 and raises it until the tungsten alloy inserts 26 are in register with the windows 11a and 12a on opposite sides of the source unit 10. When the cam 21 is in the position shown in Figure 3, the shutter unit 20, is in its lower-most position, the tungsten alloy inserts 26 having been lowered so as to expose the source 13.

It will be understood that the source unit 10 is ordinarily fixedly mounted in an instrument, such as described in our application S. N. 285,348, filed April 30, 1952, and that the shutter 20 is aligned with respect to the source unit 10 by guide members (not shown).

One apparatus for depositing our radioactive sources is shown in Figure 5. It comprises a brass base plate 30 and a brass top plate 31 about 2 inches square. Threaded pins 32 are provided in the corners of the plates. A rubber gasket 33 supports the mica sheet or gold foil 14 over which is pressed a teflon (polytetrafluorethylene) block 34. The block 34 has a conical cavity 35 which tapers from about 1 inch at the top face to about inch at the lower face of the block. A port 36 in the top plate 31 is in register with the upper end of the conical cavity 35.

The jig is so arranged that the metallic foil or mica sheet 14 forms the bottom of the conically shaped cavity 35 which is the solution holder. After the sheet 14 has been mounted and the solution poured into place the jig is placed under an infrared lamp (not shown) and the solution slowly evaporated to dryness. When the evaporation is complete, the radioactive salts are uniformly deposited on the sheet 14 in a circular area determined by the bottom dimension of the conical solution holder 35. If desired, an air jet or other means can be provided for forcing the nonevaporated portion of the solution to flow down the face of the conical solution holder into the desired area on the base sheet 14. Likewise, a pre-formed pellet of radioactive material can be placed directly between the base and cover sheets.

In the construction of the beta ray sources, according to our invention, a typical solution includes 1.70 millicures strontium (in equilibrium with yttrium 90) chloride per milliliter of solution in weak HCl having a total solid content of 1.1 milligrams per milliliter.

A measured amount of this solution is evaporated on a thin base sheet 14, e. g., gold foil, to produce a uniform layer of dry salt over a desired area of the base sheet. The measured amount of the solution is placed in the conical container 35 and the assembly is placed under an infrared lamp where the solution is slowly evaporated and the solids finally deposited over that area of the base sheet which is exposed by the lower opening in the tefion block. In the illustrated embodiment the cavity in the tefion block has a capacity of about 2 milliliters of solution and the area on the base sheet over which the salts are deposited is from 0.125 to 0.25 inch in diameter. The deposit of inactive plus active material should not be more than mg./cm. This can be obtained by using A. E. 6. high purity carrier-free material.

The foil or mica sheet is removed from the jig, and a second piece of foil placed on top of the first, with a radiation-resistant adhesive between them. The two sheets are pressed together to form a sandwich with the radioactive salt centrally located between the base and top sheets and the unit held together between the annular discs 11 and 12 by screws 1.6. The whole assembly is heated on a hot plate until the mica and polyethylene sheets bond together. After cooling, the screws and edges of the brass discs 11 and 12 can be coated with an air drying cement to provide a peripheral seal.

The source 13 has an activity of from about 3 me. to about 10 me. when prepared for use in the apparatus described in our copending application S. N. 285,348.

Although we have described the invention in terms of a specific example, it is to be understood that this is by Way of illustration only and that our invention is not limited thereto. Alternative embodiments will become apparent to those skilled in the art in view of our disclosure and, accordingly, modifications of the invention are contemplated without departing from the spirit thereof.

Having thus described our invention, we claim:

1. A radioactive source comprising a pair of annular disc-like members, a pair of sheets interposed said disclike members, a quantity of radioactive material between central portions of said sheets, means for securing said disc-like members and sheets in a compact unit, and centrally disposed apertures in each of said disc-like members in register with said quantity whereby beta rays are emitted from opposite faces of the radioactive source simultaneously.

2. A radioactive source comprising a fiat casing means formed of a material not readily disintegrated by prolonged bombardment from radioactive materials, a source support means in said casing means, said support means consisting essentially of a pair of confronting base sheets transparent to the spectrum of radiation from said deposit of radioactive material, a deposit of radioactive material on said support, an annular plastic layer between said base sheets and encircling said deposit, and opposed windows in said casing means.

3. An enclosed radioactive source comprising a pair of annular disc-like members, centrally disposed windows in said members, a pair of mica sheets interposed said disclike members, a beta ray source material on a central portion of at least one of said mica sheets in register with said windows whereby beta rays are emitted from opposite faces of the radioactive source simultaneously, and a polyethylene sheet between said mica sheets, said polyethylene sheet having a central opening to accommodate the said source material.

4. A radioactive source comprising a pair of separable annular disc-like members, a pair of sheets of mica interposed said disc-like members, a deposit of strontium on a central portion of one of said mica sheets, said deposit confronting the other of said mica sheets, centrally disposed apertures in said members in register with said deposit whereby beta rays are emitted from opposite faces of the radioactive source simultaneously, a polyethylene sheet between said mica sheets, said polyethylene sheet having a central opening to accommodate the said deposit, and screw means holding said members and interposed sheets as a unit enclosing said deposit.

5. A controllable radioactive source comprising in combination a unitary casing and a movable shutter means for said casing, said casing including a pair of annular disc-like shielding members having aligned centrally located windows therein, a pair of confronting base sheets interposed said annular shielding members, a radioactive deposit on one of said base sheets and confronting the other of said base sheets, said deposit being in register with said Windows, a plastic binder layer between said base sheets, said binder layer being fiowable by heat and being of generally annular configuration to encircle said deposit, said shutter means including a pair.

of spaced plates, a spacer block fixed between said plates adjacent the bottom thereof, said spacer block and plates forming a generally U shaped unit adapted to receive said casing and to partially enclose the opposite sides thereof, a pair of aligned slots in said plates in register with the said windows of said casing and having a length approximately equivalent to the radius of said casing, a tungsten alloy shielding plug fixed in the lower portion of each of said slots, and acam means acting on the bottom of said shutter for moving said plugs radially of said casing into and out of register with said windows.

6. A radioactive source comprising in combination a unitary .casing and a movable shutter means for said casing, said casing comprising a pair of flat shielding members, a pair of confronting sheets interposed said shielding members, a quantity of radioactive material between central portions of said sheets, a binder layer between said confronting sheets, centrally disposed apertures in each of said shielding members in register with said quantity of radioactive material whereby beta rays are emitted from opposite faces of the radioactive source simultaneously, said shutter means including a pair of spaced plates, said plates confining said casing therebetween, a pair of aligned slots in said plates in register with the said apertures in said casing, a radiation shielding plug fixed in each of said slots, and a cam means acting on said shutter for moving said plugs into and out of register with said aligned apertures.

Feuer et al July 30, 1946 Schultz et al Nov. 13, 1951 

